Projection systems used for projecting an image on a screen can use multiple color light sources, such as light emitting diodes (LED's), with different colors to generate the illumination light. Several optical elements are disposed between the LED's and the image display unit to combine and transfer the light from the LED's to the image display unit. The image display unit can use various methods to impose an image on the light. For example, the image display unit may use polarization, as with transmissive or reflective liquid crystal displays.
Still other projection systems used for projecting an image on a screen can use white light configured to imagewise reflect from a digital micro-mirror array, such as the array used in Texas Instruments' Digital Light Processor (DLP®) displays. In the DLP® display, individual mirrors within the digital micro-mirror array represent individual pixels of the projected image. A display pixel is illuminated when the corresponding mirror is tilted so that incident light is directed into the projected optical path. A rotating color wheel placed within the optical path is timed to the reflection of light from the digital micro-mirror array, so that the reflected white light is filtered to project the color corresponding to the pixel. The digital micro-mirror array is then switched to the next desired pixel color, and the process is continued at such a rapid rate that the entire projected display appears to be continuously illuminated. The digital micro-mirror projection system requires fewer pixelated array components, which can result in a smaller size projector.
LED illumination is becoming a common method for projection illumination. LEDs offer long life, high color gamut, high efficiency, the ability to be strobed for sequential imagers, and contain no mercury. However, LEDs have a relatively low brightness. One way of at least doubling the effective brightness of a white source made from red, green, and blue LEDs is to use a color combiner, which uses dichroic filters to make the individual colors of LEDs optically appear to spatially overlap with each other. These types of devices are broadly described as being “color combiners”.
Color combiners typically use dichroic filters that are tilted relative to the light beams passing through them. 3M Company has recently developed color combiners where the dichroic filters are at normal incidence angles to the average light path for the LED output, and the light is effectively diverted through a combination of a reflective polarizer and quarter wave plates.
Image brightness is an important parameter of a projection system. The brightness of color light sources and the efficiencies of collecting, combining, homogenizing and delivering the light to the image display unit all affect brightness. As the size of modern projector systems decreases, there is a need to maintain an adequate level of output brightness while at the same time keeping heat produced by the color light sources at a low level that can be dissipated in a small projector system. There is a need for a light combining system that combines multiple color lights with increased efficiency to provide a light output with an adequate level of brightness without excessive power consumption by light sources. There is also a need for a light combining system that directs light of different wavelength spectra in a manner to minimize the degradation of the wavelength-sensitive components in the light combiner.